Concrete is the most important construction material in the world. It is composed of stone and sand aggregate bound by a cementitious binder. The cementitious binder is manufactured by grinding Portland cement clinker with calcium sulphate with or without additional supplementary cementitious materials or other clinker replacement materials such as limestone (henceforth limestones are also included as supplementary cementitious material). Portland cement clinker is produced through the reaction of limestone with aluminous and ferrous raw materials at 1450-1500° C. in a rotary kiln. The energy consumption needed to heat the material to this high temperature combined with the chemical decomposition (decarbonation) of limestone which liberates CO2 to the atmosphere results in an emission of typically 0.8 kg CO2 per kg clinker produced.
The increasing demand for housing, civil works and industrial buildings in the developing parts of the world has led to a sharp rise in demand for cement. It is expected that the global cement production in 2050 will be more than double the 2010 level. Therefore, there is an urgent need to increase the cement production capacity whilst at the same time reducing the CO2 emissions associated with the production of Portland cement clinker.
An effective way of increasing cement capacity whilst simultaneously reducing CO2 emissions is to replace part of the clinker by supplementary cementitious materials such as fly ash or ground granulated blastfurnace slag. However, the use of supplementary cementitious materials is limited by the availability of suitable materials and technical constraints in the present art which in turn limits the content of clinker which may be substituted.
Portland-limestone cements are widely used in many parts of the world including Europe where they are classified according to EN 197-1:2000 as CEM II/A LL, which apart from clinker as the main constituent can contain up to 20 weight % limestone, or CEM II/B LL cements, which apart from clinker can contain up to 35 weight % limestone.
As a rule, the substitution of Portland cement clinker in the ground cement with limestone of similar or high surface area results in lower strengths since most of the limestone does not react and its effect is mainly one of dilution.
One notable exception is when the limestone is used in conjunction with mineralised clinker as taught in the European patent EP 0 640 062 B1 where higher strengths are obtained than would predicted by the dilution effect on its own. However, even in this case there is a limit to how much limestone can be added to the cement (generally in the region of 10 to 15% clinker replacement) before strengths are significantly reduced. It is of course important when evaluating the information disclosed by EP 0 640 062 B1 or elsewhere in the literature that other factors which may effect strengths such as the fineness of the Portland cement clinker fraction after grinding, or the water to binder ratio (in this case water/(clinker+limestone)) are kept constant. Despite the largely diluting effect of limestone on the standard mortar strengths Portland limestone cements account for approximately a quarter of all cements sold in Europe where it is used to produce relatively low strength concrete and where its main role is to achieve the required cement contents in the concrete mix needed for optimum rheology.
Limestone addition therefore plays an important role in significantly reducing CO2 emissions from clinker production associated with the production of about a quarter of the concrete produced in Europe. However, for the remaining concrete where the contents of other types of Portland cements needed to achieve the concrete strengths specified for a given application are sufficient for optimum rheology and/or specified by the relevant concrete standards Portland limestone cement are generally unsuitable.
For higher strength concretes alternative clinker replacement materials to limestone are needed to reduce the cost of cement production and lower CO2 emissions. A prerequisite for these materials is that they contribute to strength development. Two major classes of these materials exist in the European cement standard, i.e. granulated blastfurnace slag (GBFS), and natural or artificial pozzolans including siliceous fly ash as the most important material in the latter class. Unfortunately, however, the availability of these materials is limited. In Europe for example waste materials such as GBFS and siliceous fly ash are almost fully utilised either in cement production or added directly to the concrete, whilst natural pozzolans such as volcanic ash are not evenly distributed geographically.
One approach to resolve this dilemma is to produce another class of pozzolans by heating clay to between 500° C. and 800° C. as for example taught in the U.S. Pat. No. 5,626,665. Unfortunately, the reactivity of the calcined clay varies according to clay type (Mielenz, R. C., Witte, L. P. & Glantz, O. J. (1950). STP-99, American Society for Testing of Materials. Philadelphis. 43-91. Only mixtures with kaolin obtain strength comparable to pure Portland cement. Intermediate results are obtained by using calcined smectite type clays, whereas illite exhibits limited reactivity.
The result of this is that for most types of clays found in sufficient amounts the contribution to strength has not been satisfactory.
U.S. Pat. No. 4,737,191 describes a product comprising an intimate mixture of clay and limestone which is heat treated in a CO2 atmosphere at 700-900° C. whereby a chemical reaction between limestone and clay is promoted. Thus, a chemical reaction occurs between the clay minerals and the carbonate. However, the strength reported is significant lower than a state-of-the art pure Portland cement.
U.S. Pat. No. 1,521,967 relates to a dry mortar mixture containing sand, Portland cement, clay and limestone. Prior to mixing of the components, the pulverised limestone is heated to 1200° F. (650° C.) and the clay is heated to 900° F. (482° C.; column 2, lines 69-84). The purpose of heating the clay to this comparatively low temperature is merely stated as drying the material. Significantly, U.S. Pat. No. 1,521,967 is silent on mortar strength of the obtained material.
European Patent Application EP 0 895 972 A1 relates to an alkaline aluminoferrosilicate hydraulic cement which may contain Portland cement clinker, metakaolin and dolomite sintered at 800-950° C. As acknowledged in EP 0 895 972 A1, the high temperature heat treatment leads to a full decomposition and decarbonation of dolomite to CaO and MgO.
U.S. Pat. No. 6,030,447 discloses a formulation for preparing an autoclave cured cementitious material, said material comprising a cementitious material such as Portland cement and/or lime (CaO), a siliceous material such as ground sand, and a dehydroxylated clay mineral such as metakaolin. U.S. Pat. No. 6,030,447 is concerned with improving water permeability of the obtained material and is thus silent on CO2 reduction potential in relation to standard cement strength.
Thus, there is a need in the art for cements having a satisfactory strength but which use less energy and emit lower amounts of CO2 when produced.
It is therefore one object of the invention to provide a cement at lower net CO2 emissions having the same level of concrete performance.
It is another object of the present invention to provide a cement with a comparatively high content of supplementary cementitious material, which cement retains a high compressive strength.